1. An optoelectronic sensor for detecting objects in a monitored zone, said optoelectronic sensor having the following:

a front screen;

a light transmitter for transmitting a light beam;

a movable deflection unit for a periodic sampling of the monitored zone by the light beam;

a light receiver for generating a received signal from the light beam remitted by the objects;

at least one test light transmitter, at least one test light receiver and at least one test light reflector which span a test
light path through the front screen;

and an evaluation unit which is configured to acquire pieces of information on the objects present in the monitored zone from
the received signal and to recognize an impaired light permeability of the front screen from a test light signal which the
test light receiver generates from test light which is transmitted from the test light transmitter and which is reflected
at the test light reflector,

wherein the test light reflector is arranged such that it moves along with the deflection unit.